Synthesis of novel chiral ionic liquid and its application in reduction of prochiral ketones to the corresponding chiral alcohols using NaBH4

Article abstract of DOI:10.1080/00397910903161801

A novel chiral ionic liquid (CIL) based on nicotinium salt has been synthesized and used as an efficient asymmetric chiral catalyst for reduction of acetophenone derivatives with NaBH₄ in methanol at room temperature. The optically active alcoh

Full text of DOI:10.1080/00397910903161801

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Synthesis of Novel Chiral Ionic Liquid and
Its Application in Reduction of Prochiral
Ketones to the Corresponding Chiral
Alcohols Using NaBH
4
a b
b
Abdol Reza Hajipour
& Fatemeh Karimi
a
Department of Pharmacology , University of Wisconsin Medical
School , Madison, Wisconsin, USA
b
Pharmaceutical Research Laboratory, Department of Chemistry ,
Isfahan University of Technology , Isfahan, Iran
Published online: 17 May 2010.
To cite this article: Abdol Reza Hajipour & Fatemeh Karimi (2010) Synthesis of Novel Chiral Ionic
Liquid and Its Application in Reduction of Prochiral Ketones to the Corresponding Chiral Alcohols Using
NaBH , Synthetic Communications: An International Journal for Rapid Communication of Synthetic
4
Organic Chemistry, 40:12, 1784-1793, DOI: 10.1080/00397910903161801
To link to this article: http://dx.doi.org/10.1080/00397910903161801
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1
Synthetic Communications , 40: 1784–1793, 2010
Copyright # Taylor & Francis Group, LLC
ISSN: 0039-7911 print=1532-2432 online
DOI: 10.1080/00397910903161801
SYNTHESIS OF NOVEL CHIRAL IONIC LIQUID AND
ITS APPLICATION IN REDUCTION OF PROCHIRAL
KETONES TO THE CORRESPONDING CHIRAL
ALCOHOLS USING NaBH4
1
,2
2
Abdol Reza Hajipour and Fatemeh Karimi
1
Department of Pharmacology, University of Wisconsin Medical School,
Madison, Wisconsin, USA
2
Pharmaceutical Research Laboratory, Department of Chemistry, Isfahan
University of Technology, Isfahan, Iran
A novel chiral ionic liquid (CIL) based on nicotinium salt has been synthesized and used as
an efficient asymmetric chiral catalyst for reduction of acetophenone derivatives with
4
NaBH in methanol at room temperature. The optically active alcohols were obtained in
low to moderate enantiomeric excess in a short reaction time.
Keywords: Acetophenone; asymmetric reduction; chiral alcohols; chiral ionic liquid
INTRODUCTION
Ionic liquids (ILs) have attracted considerable attention as green reaction
media for organic synthesis, chemical engineering, electrochemistry, and materials
[
1]
science, especially in synthetic and catalytic transformation. Among these ILs,
the chiral ionic liquids (CILs) are particularly attractive for their potential applica-
tions in syntheses, chiral chromatography, NMR chiral discrimination, stereoselec-
[2]
tive polymerization, and liquid crystals. In particular, the potential of CILs as
chiral induction catalyst for asymmetric reactions has recently prompted researchers
to synthesize such materials. Optically active compounds are important building
blocks for the synthesis of pharmaceuticals, fragrances, pheromones, pesticides,
[
3]
and advanced materials such as liquid crystals and flavors. It is known that the
enantioselective reduction of prochiral ketones is an important method for prep-
[
4]
aration of optically active secondary alcohols. The optically active alcohols are
the most important chiral synthons for agrochemical and pharmaceutical
[
5]
industries.
Received April 22, 2009.
Address correspondence to Abdol Reza Hajipour, Pharmaceutical Research Laboratory,
Department of Chemistry, Isfahan University of Technology, Isfahan 84156, Iran. E-mail: haji@
cc.iut.ac.ir
1
784

PREPARATION OF CHIRAL ALCOHOLS
1785
Scheme 1. Synthesis of CIL based on nicotinium salt.
RESULTS AND DISCUSSION
In continuation of our research to develop new methods in organic
[
6–10]
synthesis,
here we report the synthesis of CIL and its application as a novel
chiral induction catalyst for reduction of acetophenone derivatives to the corre-
sponding chiral alcohols in methanol at room temperature. The CIL was synthesized
from (–)-nicotine hydrogen tartrate salt, sodium carbonate, and 1-bromobutane in
methanol under reflux conditions in 85% yields (Scheme 1). The CIL was character-
1
ized by spectroscopic data ( H NMR and IR).
Various acetophenone derivatives were reduced to the corresponding alcohols
in the presence of (ꢀ)-N,N-dibutylnicotinium bromide as a CIL. Initially, the opti-
mum reaction conditions were found by reduction of acetophenone under various
experimental conditions. As shown in Table 1, the methanol gave the best results
(
entries 1–4) and the optimized ratio of the CIL was 10 mol% (Table 1, entries
–7). We also investigated the effect of the molar ratio of ketone and NaBH , and
5
the results showed that the molar ratio of 1:1.5 for ketone=NaBH gave the best
4
4
results for the reduction of acetophenone (Table 1, entries 8–10).
Table 1. Determination of optimized conditions for asymmetric reduction of acetophenone with NaBH
in the presence of CIL
4
a
Entry
Acetophenone=NaBH
4
CIL (mol%)
Time (min)
Ee
Solvent
1
2
3
4
5
6
7
8
1:1.5
1:1.5
1:1.5
1:1.5
1:1.5
1:1.5
1:1.5
1:1.25
1
1
20
60
60
—
60
20
15
60
—
—
—
MeOH
EtOH
1
—
—
2
H O
1
2 2
CH Cl
1
17.40
28.35
52.52
—
MeOH
MeOH
MeOH
MeOH
MeOH
5
10
10
10
1
:1
—
a
Determined by chiral GC analysis.

1786
A. R. HAJIPOUR AND F. KARIMI
Table 2. Enantioselective reduction of acetophenone derivatives with NaBH
4
in the presence of CIL
a
b
Entry
1
Substrate
Product
Time (min)
Ee (%)
45
Configuration
20
R
2
3
5
5
49
52
R
R
4
5
6
10
27
65
47
R
R
R
5
30
7
8
20
25
32
27
R
R
(
Continued )

PREPARATION OF CHIRAL ALCOHOLS
1787
Table 2. Continued
a
b
Entry
9
Substrate
Product
Time (min)
20
Ee (%)
50
Configuration
R
R
1
1
0
1
50
40
32
24
R
R
12
5
61
—
1
3
4
—
—
R
R
1
—
a
b
Determined by chiral GC analysis.
Absolute configuration determined by comparison of the optical rotation with literature
[
3,5,11–13]
value.
Several acetophenone derivatives were reduced using a catalytic amount of
0 mol% with molar ratio of 1:1.5 for ketone=NaBH in methanol at room tempera-
4
1
ture in short reaction times with good to moderate ee (enantiomer excess) (Table 2).
The ee values were determined by chiral gas chromatography (GC), and the absolute
configuration was determined by comparison of the optical rotation with literature
[3,5,11–13]
value as demonstrated in Table 2.
The reduction time for ketones with
electron-withdrawing groups was less than for ketones with electron-releasing
groups (entries 2–5). These observations are in agreement with the reduction
mechanism with addition of hydride ion as nucleophile. Also, the results show that
this method is not suitable for preparation of chiral alcohols from ketones with
electron-releasing groups such as OH and NH in the benzene ring (entries 13
2
and 14). The chiral products were characterized by spectroscopic analysis such as
1
H NMR and IR.
To evaluate the efficiency of this CIL, we also investigated the reaction in the
presence of a catalytic amount of (–)-nicotine hydrogentartrate salt. The reduction of

1788
A. R. HAJIPOUR AND F. KARIMI
Table 3. Enantioselective reduction of acetophenone derivatives with NaBH
–)-nicotine hydrogen tartrate salt
4
in the presence of
(
a
b
Entry
1
Substrate
Product
Time (min)
35
Ee (%)
11
Configuration
R
2
3
5
5
23
18
R
R
4
10
19
R
5
6
15
25
23
21
R
R
7
8
30
35
10
19
R
R
(
Continued )

PREPARATION OF CHIRAL ALCOHOLS
1789
Table 3. Continued
a
b
Entry
9
Substrate
Product
Time (min)
Ee (%)
Configuration
25
12
R
R
1
1
0
1
55
50
23
7
R
R
12
10
—
43
1
1
3
4
—
—
—
—
—
a
b
Determined by chiral GC analysis.
Absolute configuration determined by comparison of the optical rotation with the literature
[
3,5,11–13]
value.
acetophenone derivatives was performed with a molar ratio of 1:1.5 for ketone and
NaBH in the presence of 10 mol% of (–)-nicotine hydrogentartrate salt in methanol
4
at room temperature. The reaction times were from 5 to 55 min, and ee of the
products were less than when using CILs (7–43%), which may be due to greater solu-
bility of the reaction mixture when using ILs as catalysts. The results are summarized
in Table 3.
Results showed that (–)-N,N-dibutylnicotinium bromide as CIL was more
effective in chiral induction than (–)-nicotine hydrogentartrate salt with greater
optical rotation and greater ee.
CONCLUSION
In conclusion, we have prepared a CIL and successfully applied it to the asym-
metric reduction of ketones with moderate to good enantioselectivity. Straightforward

1790
A. R. HAJIPOUR AND F. KARIMI
procedure, short reaction time, mild reaction conditions, and easy isolation of the
product are the advantages of this method.
EXPERIMENTAL
General
All reagents were purchased from Merck and Aldrich and used without further
1
purification. Products were identified by H NMR, IR, and melting point. H NMR
1
was recorded at 300 MHz in CDCl using tetramethylsilane (TMS) as an internal
3
standard. IR spectra were obtained using a Perkin Fourier transform (FT)–IR spec-
trometer. All melting points are uncorrected. Optical rotation values were measured
on a Jasco P-1030 polarimeter. Ee values were determined by chiral GC analysis, and
absolute configuration was determined by comparison of the optical rotation with
[3,5,11–13]
literature value.
Preparation of Chiral Ionic Liquid
A solution of 4.62 g of (–)-nicotinium hydrogentartrate (10 mmol), 2.12 g of
sodium carbonate (20 mmol), and 2 ml 1-bromobutane (20 mmol) in 10 ml of meth-
anol in a round-bottom flask was refluxed. The progress of the reaction was followed
by TLC (chloroform–methanol, 50:50). After completion of the reaction (4 h), the
reaction mixture was filtered and the solvent was evaporated under reduced pressure
2
5
to give (–)-N,N-dibutyl nicotinium bromide in 85% yields. ½aꢁ ¼ ꢀ137.2 (c ¼ 0.005,
D
MeOH); H NMR (CDCl ): 7.4–8.6 (m, 4H), 5 (t, 2H), 3.7 (m, 1H), 3.5 (m, 2H), 2.5
1
3
(t, 2H), 2.4 (s, 3H), 1.9–2.2 (m, 4H), 1.8 (m, 4H), 1.4 (m, 4H), 0.9 (t, 6H).
Typical Procedure for Asymmetric Reduction of Prochiral Ketones to
Chiral Alcohols in the Presence of CIL
A solution of 0.12 g acetophenone (1 mmol), 0.0436 g (10% mol) CIL, and
.0567 g (1.5 mmol) of NaBH in 5 ml MeOH in a round-bottom flask was stirred
4
0
at room temperature. The reaction progress was followed by thin-layer chromato-
graphy (TLC; EtOAc–cyclohexane, 25:75). After completion of the reaction, the
product was extracted with CH Cl (2 ꢂ 5 ml) and washed with water and HCl
2
2
5
%. The organic layer was dried with MgSO to give the pure product without
4
further purification.
2
D
5
1
(
R)-1-Phenylethanol. ½aꢁ ¼ þ22 (c ¼ 0.005, CH Cl ); H NMR (CDCl3):
2
2
1
.50 (d, 3H, CH ), 1.88 (s, 1H, OH), 4.90 (q, 1H, CH), 7.25–7.39 (m, 5H, Ph).
3
2
5
1
(
R)-1-(4-Nitro-phenyl)-ethanol. ½aꢁ ¼ þ19.5 (c ¼ 0.005, CH Cl ); H NMR
D
CDCl ): 8.10 (d, 2H), 7.48 (d, 2H), 4.97 (q, 1H, CH), 2.80 (s, 1H, OH), 1.46 (d, 3H,
2
2
(
CH3).
3
2
D
5
1
(
R)-1-(4-Fluoro-phenyl)-ethanol. ½aꢁ ¼ þ24.1 (c ¼ 0.005, CH Cl );
H
NMR (CDCl ): 7.23 (d, 2H), 6.96 (d, 2H), 4.74 (q, 1H, CH), 3.64 (s, 1H, OH),
2
2
3
1
.36 (d, 3H, CH3).

PREPARATION OF CHIRAL ALCOHOLS
1791
2
D
5
1
(
R)-1-(4-Chlorophenyl)-ethanol. ½aꢁ ¼ þ34.2 (c ¼ 0.005, CH Cl );
H
NMR (CDCl ): 1.47 (d, 3H, CH ), 1.89 (s, 1H, OH), 4.88 (q, 1H, CH), 7.29–7.34
2
2
3
3
(m, 4H, Ph).
2
D
5
1
(
R)-1-(4-Bromophenyl)-ethanol. ½aꢁ ¼ þ10.9 (c ¼ 0.005, CH Cl );
H
NMR (CDCl ): 1.47 (d, 3H, CH ), 1.85 (s, 1H, OH), 4.87 (q, 1H, CH), 7.23–7.49
2
2
3
3
(m, 4H, Ph).
2
D
5
1
(
R)-1-(2-Methoxyphenyl)-ethanol. ½aꢁ ¼ þ14.1 (c ¼ 0.005, CH Cl );
H
NMR (CDCl ): 7.35–6.88 (m, 4H, Ph), 5.10 (q, 1H, CH), 3.86 (s, 3H, OCH ),
2
2
3
3
2
.74 (s, 1H, OH), 1.51 (d, 3H, CH3).
2
D
5
1
(
R)-1-(3-Methoxyphenyl)-ethanol. ½aꢁ ¼ þ18.8 (c ¼ 0.005, CH Cl );
H
NMR (CDCl ): 1.49 (d, 3H,CH ), 1.85 (s, 1H, OH), 3.82 (s, 3H, OCH ), 4.88 (q,
2
2
3
3
3
1
H, CH), 6.80–7.29 (m, 4H, Ph).
2
D
5
1
(
R)-1-(4-Methoxyphenyl)-ethanol. ½aꢁ ¼ þ14.1 (c ¼ 0.005, CH Cl );
H
NMR (CDCl ): 7.33 (d, 2H), 6.90 (d, 2H), 5.10 (br, 1H, OH), 3.85 (s, 3H, OCH ),
2
2
3
3
2
.88 (s, 1H), 1.49 (d, 3H).
2
5
1
(
R)-1-Phenyl-2,2,2-trifluoroethanol. ½aꢁ ¼ þ14.4 (c ¼ 0.005, CH Cl ); H
D
NMR (CDCl ): 2.81 (s, 1H,OH), 5.03 (q, 1H, CH), 7.40–7.50 (m, 5H, Ph).
2
2
3
Typical Procedure for Asymmetric Reduction of Prochiral Ketones to
Chiral Alcohols in the Presence of (–)-Nicotine Hydrogentartrate Salt
A solution of 0.12 g of acetophenone (1 mmol) in 5 ml MeOH, 0.0462 g (10%
mol) CIL, and 0.0567 g (1.5 mmol) of NaBH in a round-bottom flask was stirred
4
at room temperature. The progress of the reaction was followed by TLC (EtOAc–
cyclohexane, 25:75). After completion of the reaction, the product was extracted
with CH Cl (2 ꢂ 5 ml) and washed with water. The organic layer was dried with
2
2
MgSO to give the pure product without further purification.
4
ACKNOWLEDGMENTS
We gratefully acknowledge the funding support received for this project from
the Isfahan University of Technology (IUT), Iran. Further financial support from
Center of Excellence in Sensor and Green Chemistry Research (IUT) is gratefully
acknowledged.
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